Prime Minister John Key and his Samoan counterpart have signed a double-tax agreement in Samoa.

The announcement comes on the day of the All Blacks' first test match on Samoan soil.

The double-tax agreement will help reduce tax barriers to two-way trade and investment by preventing cross-border income being taxed twice.

Mr Key said it would help further trade relationships between the two countries and acknowledged it would be particularly beneficial for Samoa.

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"New Zealand is Samoa's second largest trading partner. The agreement will provide a platform for increased trade and investment between our two countries - and will help assist the economic development of our Pacific Island neighbour."

The new tax agreement will come into play later this year.